The invention relates to flux compositions for protecting and further refining molten steel in a tundish. The tundish fluxes are particularly useful during the transfer of steel from the tundish to the caster mold in a continuous casting process. However, the compositions can be used in any steelmaking process requiring a tundish.
Historically, steel production utilized blast furnace iron and a scrap charge in a Basic Oxygen Furnace (BOF) or scrap melting in an electric arc furnace to produce ingots of cast steel for reheating and rolling into manufacturing stock. Increasingly demanding applications have led to the development of more stringent physical and chemical specifications for the final steel products.
The ladle metallurgy furnace (LMF) is an additional steel refining step that has become a widely used tool to ensure consistent conformance to the rigid steelmaking requirements set by continuous casters. This additional refining step employs a ladle slag to lower the level of elements, such as sulfur and phosphorous, maintain or lower the oxygen level, and decrease the content of non-metallic inclusions, such as alumina and various sulfide and oxide species. The ladle slag composition is designed for the different grades of steel being produced, with the majority requiring desulfurization. A ladle slag is best utilized for refining when it is a fluid and vigorously mixed with the steel through dynamic physical particle interaction. Thus, it is advantageous for the ladle slag to become as fluid as possible immediately upon tapping the metal from the furnace into the ladle. The full body of the separate slag and metal masses may then enter the slag/metal reaction interface where chemical refining is most rapid. Fluidity and slag/metal mixing in the ladle therefore effectively increases the interfacial area of the slag to accelerate the refining reactions. Rapid and efficient chemical refining of steel in the ladle also requires large quantities of lime in solution within the slag to provide the high basicity needed for maximum sulfur and phosphorus transfer between the steel and the slag. The reaction at the slag/metal interface between calcium oxide in the slag and dissolved sulfur in the molten steel produces calcium sulfide that remains stable within the top slag layer as long as a reducing chemistry is maintained.
Following the refining step in the LMF, the molten steel is poured from the ladle into a tundish from which the molten steel passes to the continuous casting mold. The tundish is required as an intermediary between the ladle and the caster mold to act as a reservoir and thus facilitate the continuous supply of steel to the caster. In order to protect the molten steel from deleterious changes to chemical and thermal profiles during the residence time in the tundish, it is known to apply a layer of powder to the surface of the molten steel in the tundish. The applied powder melts upon its addition to the tundish due to heat transfer from the steel, thus creating a liquid slag layer at the molten steel interface. This tundish slag layer not only protects the liquid steel from atmospheric oxidation, but further facilitates any additional or final refining of the steel prior to solidification during casting. The tundish slag layer also serves to absorb any ladle slag carryover from the ladle being tapped into the tundish. In contrast to the mixing of the slag with the molten steel in the ladle, the tundish slag remains as a surface layer.
The tundish slag (flux) is required to have the following functions and properties:
1) To easily form a continuous surface layer on the liquid steel to provide complete atmospheric protection and thus prevent reoxidation of metals in the steel and its alloys. PA1 2) To provide thermal insulation to prevent the molten steel from losing heat. PA1 3) To melt quickly at the molten steel interface to allow immediate absorption of nonmetallic inclusions, such as alumina, silica or magnesia, which may be present in the liquid steel. PA1 4) To remain liquid at the metal interface, even after considerable alumina has been absorbed, so as to continue to function as a continuous layer for thermal insulation and inclusion absorption. PA1 5) To be restricted in content of highly corrosive fluxing elements (such as fluorine, lithium, sodium, potassium, titanium and boron) to prevent or minimize refractory erosion within the tundish. PA1 6) To contain a small amount of magnesia to retard the rate and extent of the refractory erosion in the lining of the tundish and the flow control devices contained within. PA1 7) To contain low levels of easily reducible oxides, such as FeO and MnO, to avoid reoxidation of the steel and its alloys. PA1 8) To be restricted in the level of silica contained within the mix to prevent or minimize the pickup of silicon by the steel in grades of steel that are silicon restricted. PA1 9) To have some desulfurizing capability in order to allow for the cleanest and highest quality steels to be cast into final products with the most desirable mechanical properties. PA1 10) To be cost effective for application to the widest range of steel grades for maximum benefit to steelmakers of both lower quality (e.g. rebar) and high quality (e.g. critical exposed) steels.
To facilitate the spreading of the flux over the entire surface of the steel in the tundish, several approaches have been described. Materials which contain a certain L.O.I. (Loss On Ignition) have been incorporated into final flux product blends in order to provide a gas layer that is generated at the interface between the steel and the tundish flux layer. This gas layer acts as a low friction surface for increased material flowability to quickly and easily spread the applied powder to the furthest reaches of the tundish. Such L.O.I. materials include calcium carbonate, magnesium carbonate, sodium carbonate, wood flour, powdered coke, graphite, and unburned ricehulls. Typically, these materials are used in percentages sufficient to yield a final product L.O.I. of between 4 and 10%.
For rapid melting when the flux material contacts the surface of the molten steel, low melting point fluidizing materials, such as dense soda ash and/or glass cullet, nepheline, potash, lithia and lithium carbonate, may be incorporated into the tundish flux mixture. Other materials used as fluidizers in tundish flux powders include potassium carbonate, sodium carbonate, wollastonite, feldspar, cryolite, borax, fluorspar, sodium silicate, portland cement, calcium aluminate, lithium titanate, phosphorus furnace slag and blast furnace slag. Many of these compounds are considered to have the ability to keep the flux layer fluid at the steel interface even after a considerable portion of alumina has been absorbed. However, as discussed below, many of these materials, such as fluorspar, calcium aluminate, phosphorus furnace slag and blast furnace slag, have disadvantages for use in a tundish flux mixture that outweigh their possible usefulness as fluidizing agents.
For thermal insulation of the steel surface in the tundish, burned ricehulls are often employed. Ricehulls, which typically contain 95% silica and 5% carbon, are inexpensive compared to traditional basic tundish flux compositions and provide very good thermal insulation. However, burned ricehulls are solid at steel-casting temperatures and are not very effective for oxidation protection or inclusion absorption. In addition, they are chemically acidic in nature due to the high percentage of contained silica. They have been used, however, in conjunction with basic tundish fluxes in dual applications where the basic flux composition is applied directly to the surface of the steel and the acidic, insulating layer of ricehulls is added on top of the basic flux. Burned ricehulls, however, are undesirable for use with carbon-restricted grades of steel (e.g. ultra low carbon, stainless and silicon electrical steels) or silicon-restricted grades of steel due to the likelihood of carbon or silica pickup, respectively, by the steel.
It is also necessary to minimize the level of silica in tundish flux applications in order to produce cleaner steel, wherein total oxygen contents are a measure of the final cast steel quality. It is well known that, as a weaker oxide, silica can act as an oxygen pump and effectively provide oxygen to the steel in the tundish. This, in turn, causes oxidation of alloys in the steel, such as aluminum and calcium, producing nonmetallic inclusions which cause caster nozzle clogging and quality downgrades in the cast steel product. Additionally, silica, as a nonmetallic inclusion itself, can become trapped in the steel bath, again leading to nozzle clogging and quality downgrades. Finally, as an acidic oxide, silica acts to chemically erode the basic high magnesia lining and high alumina shapes within the tundish. This chemical incompatibility within the tundish itself is a major reason for the use of tundish powders that are chemically basic in nature, rather than acidic.
Other approaches to thermal insulation of the steel surface in the tundish include the use of phosphorus furnace slag, which has been proposed as both a ladle and a tundish flux. This material is described as having the ability to only partially melt at the steel interface, while the balance of the flux layer remains as a powder to provide thermal insulation to the steel surface. However, phosphorus slag contains undesirably high levels of both silica and phosphorus, which can transfer to the steel in the tundish.
Other thermal insulation materials include acid treated graphite, which expands when heated to approximately 20 times its original volume. This material acts to effectively increase the volume of the total flux layer and creates gas space within the powder layer to provide an effective insulating layer within the volume of the covering mixture. However, graphite cannot be used for carbon restricted grades of steel.
In order to use the tundish as more than just a steel reservoir for feeding the caster, several different tundish flux formulations have been proposed to provide some measure of desulfurization to the steel as it passes through the tundish, thus using the tundish as a chemical refining vessel as well. These proposed formulations are chemically basic (high CaO, low SiO.sub.2, low FeO), have high amounts of strong fluxing materials (20-40% CaP.sub.2 and 10-70% BaO) to ensure that the lime contents are fluidized, and have 0-5% metalloids (Al, Ca or Mg) to ensure that low oxygen levels exist within the tundish flux layer. However, not only are such compositions expensive, but the high levels of fluorspar (CaF.sub.2) are also corrosive to the refractory elements in the tundish and are chemically reactive with carbon and silicon, thereby producing significant quantities of environmentally and physically harmful carbon tetrafluoride and silicon tetrafluoride. The loss of fluorine to the atmosphere via these reactions also causes a loss of fluidity in the tundish slag layer.
Apart from calcium fluoride, other low melting point materials for fluidizing higher melting point lime compounds in slag mixtures are commonly used, such as mono, di or tricalcium aluminate, with dicalcium aluminate being most preferred. Dicalcium aluminate is traditionally considered one of the lower melting point compositions of calcium aluminate available, according to the lime-alumina binary diagram where a 50/50 weight ratio of lime and alumina (12CaO.7Al.sub.2 O.sub.3 molar ratio) forms the eutectic point in the mix, with a melting point of approximately 2550.degree. F. Such high alumina flux materials, and others containing 40-70% alumina are commonly available on the market and typically used for steelmaking. However, with the incorporation of 50-75% calcium aluminate into the mixture, Al.sub.2 O.sub.3 levels in the final flux product are typically 25-50%. While this material is ideal for fluxing lime into solution within the electric furnace, the BOF, or the ladle, the alumina content is far too high to be used in a tundish slag mix, since the high alumina levels severely restrict the quantity of alumina inclusions that can be absorbed from the steel as it passes from the tundish to the mold.
There is a need, therefore, for a tundish flux composition that is inexpensive, yet overcomes the problems associated with prior tundish flux additives and meets all the requirements described above for providing a protective and refining cover for molten steel in a tundish.